1. FIELD OF INVENTION:
The present invention relates to radioactive well logging techniques to measure the lateral flow of fluid in subsurface earth formations.
2. DESCRIPTION OF PRIOR ART:
In secondary and tertiary recovery of petroleum deposits many of the recovery techniques employ the injection of water or chemical solutions into the earth formations comprising the reservoir. In planning the recovery operation, the injection of water or chemical has in the past been limited by certain assumptions and/or approximations concerning the mobility of fluids in the formation comprising the reservoir. A crucial factor in such fluid injection programs is the vertical conformity of the producing formation as well as its horizontal permeability and uniformity. In some reservoirs, formation lensing or horizontal partitioning by permeability barriers such as faults can occur. In such instances, apparently correlevant intervals of permeability may be separated from one well to another in the field by such formation lensing or permeability barriers being interposed across the interval of formation between the wells.
It is therefore apparent that large amounts of costly chemicals or water could be injected before it is established that continuity between injection and producing wells is partially or totally absent. If, on the other hand, continuity does exist, it is known that the water in the vicinity of the producing well begins to flow laterally soon after injection is initiated and long before the injected fluid actually arrives at the producing well. The detection of lateral water flow is, therefore, indicative of formation continuity. The speed of the flow, when combined with injection rates, formation thickness, formation porosity, and well spacings, can be used to determine the degree of continuity. This early definition of formation continuity could prevent the expenditure of large sums of money, time and effort in a fruitless project to recover secondary or tertiary problem deposits.
A second application of the detection of lateral water-flow is the mapping of the total flow throughout a petroleum reservoir to help in the operational planning of injecting chemicals or water and to assist in determining optimum withdrawal rates. Moreover, a knowledge of the lateral water flow characteristics of a particular formation in a producing field can help greatly in general understanding of the reservoir dynamics of the particular reservoir being produced.
It is sometime desirable in a reservoir with multiple producing intervals for a reservoir engineer to be able to delineate those producing zones which provide the most water influx or water drive to the production of petroleum. The mapping of lateral water movement in all zones both above and below the expected water table in the producing formation should supply this information to the reservoir engineer.
In the past, reservoir engineers have been provided with relatively few and often inaccurate well logging instrumentation in order to determine the vertical conformance characteristics of the earth formations comprising a reservoir. This has led to resultant confusion as to the properties of the earth formations comprising a reservoir. Radioactive tracer studies of the movement of fluids in the vicinity of a well borehole can be misleading in this respect because of the lack of uniform absorption of the tracer element into the flowing stream of formation water. Also, it is difficult to provide tracer isotopes with sufficient half life to be injected at an injection well and observe their movement days or even weeks later at a monitoring or producing well, in order to obtain some idea of the lateral flow speed or velocity of fluids in the formation comprising the reservoir.
In co-pending application Ser. No. 698,394, now U.S. Pat. No. 4,051,368 referenced above, liquids in the formation adjacent the well were bombarded with high energy neutrons. Where the fluid was at least partially saline, as in salt water, radioactive Na.sup.24 was produced by the thermal neutron capture Na.sup.23 (n,.gamma.)Na.sup.24 reaction. By observing the decrease in gamma radiation from the Na.sup.24 and 2.75 MeV or greater energy levels with time, a measure of the horizontal speed of the liquid was obtained.
Further, co-pending application Ser. No. 808,422 referenced above, discloses a method of compensating for the effect of interfering gamma radiation of the radioactive isotope manganese 56 resulting from neutron irradiation of the steel casing.
However, it has been found that gamma ray activities other than Na.sup.24 from flowing formation water behind casing and Mn.sup.56 from the casing are produced during neutron irradiation of the borehole and casing and formation adjacent thereto. Further, it has been found that these other gamma ray activities must be considered as interfering activities and compensation must be made for their presence in order to obtain more accurate, and thus more meaningful, horizontal water flow velocities in formations.
Examples of these other interfering gamma radiation activities typically present in irradiated formations include that of calcium 49 from the activation of calcium within the formation and additional sodium 24 resulting from the activation of interstitial saline formation water or from saline water which is often used to mix the borehole casing cement or saline borehole water. This additional sodium 24 activity is distinguished from the activity due to the horizontal flow of formation fluid in that the radioactive sodium casing the additional activity is not moving with the flowing formation water. It is referred to as "fixed" sodium 24 activity.